About 40% of disused satellites burning up in Earth's atmosphere these days belong to SpaceX's Starlink megaconstellation, according to Andrew Bacon, the chief technology officer and co-founder of U.K.-based in-orbit manufacturing firm Space Forge.

These satellites are mostly made of aluminum, which turns into aluminum oxide when burned at high temperatures, such as those experienced by satellites hurtling through Earth's atmosphere at orbital speeds. This aluminum ash accumulating at high altitudes worries scientists because of its potential to damage Earth's protective ozone layer and alter its albedo — the ability to reflect sunlight — which could result in changes in the temperature of the upper atmosphere.